Comparable filter devices (see, e.g., FR-A-1 415 943 and U.S. Pat. No. 3,000,505) are known in a plurality of embodiments and sizes. Within the filter housing, generally at least one hollow-cylindrical filter element removes impurities from fouled liquid supplied through an inlet into the filter housing. Flow takes place through the respective filter element from the outside to the inside. The filtering filter mat of the filter element is supported along a support tube provided with passages. The liquid which has been filtered in this way is then drained through an outlet in the filter housing for further use. The longer the filter element is in operation for filtration, the more it is clogged by dirt until finally the filter element forms an almost impassable barrier for the fluid flow (medium) and the fluid flow no longer reaches the outlet side of the filter housing. To avoid interruption of the fluid flow, which may result in the shutdown of an entire hydraulic system, for example, DE 195 46 440 A1 has suggested providing within the filter device with the filter housing a bypass valve as a bypass device. The bypass device has a spring-loaded closing part which finally opens with increasing fluid pressure within the filter housing and opens a fluid-carrying path with disconnection of the filter element directly from the inlet to the outlet. The fluid or the medium consequently no longer flows through the filter element with a definable radial direction, but moves outside the filter element essentially in the axial flow direction in the space between the outside periphery of the filter element and the inside periphery of the filter housing. The fluid flow deflected in this way then passes via the bypass valve without a filtration process through the interior of the filter element and is discharged to the outlet point. These bypass valves or other comparable bypass device are accordingly used for operating reliability and to prevent unwanted pressure rises. Unwanted pressure rises can otherwise lead to damage within the hydraulic system.
U.S. Pat. No. 5,888,383 discloses a generic filter device with a filter housing and a first filter element located in it. The first filter element extends along the middle lengthwise axis of the filter housing, through which flow of a medium takes place with a definable direction from one fluid inlet to one fluid outlet. A bypass device (bypass valve) is provided in addition to the first filter element along with another, second filter element. The respective filter elements are located in succession in the flow direction. When the bypass device takes effect, the other filter element which directly follows in the flow direction undertakes filtration of the medium. The first filter element extends essentially over the entire structural length of the filter housing. The second filter element projects with its end cap with a reduced overall length into the one filter element.
The embodiment of FIG. 2 of U.S. Pat. No. 5,888,383 moreover discloses a first filter element with high filter mesh fineness and a second filter element projecting into it with a coarser filter mesh fineness. This arrangement results in the first filter element lying to the outside primary filtration for fine filtration undertaken. Only when the first filter element has been clogged by dirt does the bypass device (bypass valve) open for the second filter element following in the flow direction to undertake coarse filtration of the medium such that in any case damage to the following hydraulic circuit is precluded.
In the known approach, the installation length of the second filter element extends essentially over the entire length of the first filter element. The outside periphery of the second filter element on the filter mat structure side is surrounded by a closed cylindrical protective jacket. On its bottom side, the protective jacket faces the fluid inlet and outlet which in the axial direction toward the middle lengthwise axis of the filter housing enter the latter on the bottom side. The second filter element has a radial distance over which the fluid to be filtered, originating from the bypass device (bypass valve), is delivered to coarse filtration by the second filter element. The indicated protective jacket stiffens the second filter element so that it is for the most part stable relative to pressure peaks, especially also relative to the fluid inlet side. Otherwise, the indicated protective jacket prevents unwanted washing out of the dirt particles collected in the second filter element onto the clean side of the filter device. Solely due to the protective jacket and the axial alignment of the fluid inlets parallel to the middle lengthwise axis of the filter housing unfavorable flow conditions arise. This arrangement adversely affects filtration performance with the known device in normal filter operation. The attachment of the protective jacket requires on the one hand installation space within the filter device and on the other hand leads to corresponding additional costs in production and maintenance.